1. Field of the Invention
The present invention relates to a method for producing a vicinal dithiol which is useful for the production of optical materials such as plastic lenses, prisms, fiber optics, information recording media, filters and adhesives for optical devices.
2. Description of the Prior Art
Cured materials produced by the polymerization of an episulfide compound added with a sulfur-containing compound such as thiol compounds are widely used at present in the applications for spectacle plastic lenses because of their good balance between the refractive index and Abbe's number. The vicinal dithiol, above all, has a high sulfur content and is extremely suitable for such applications because of the ability of increasing the refractive index of cured materials and the ability of increasing the crosslinking density of resultant polymers.
In many of the proposed methods of producing the vicinal dithiol, a vicinal diol or a vicinal dihalide is reacted with thiourea. However, the proposed reactions involve side reactions such as polymerization, rearrangement, etc. and the vicinal dithiol is produced in low yields. In other proposed methods, a compound having a thiirane ring is reacted with hydrogen sulfide or an alkali metal hydrogen sulfide. However, these proposed methods also involve undesirable polymerization, etc. and give the vicinal dithiol only in low yields. For example, the reaction of ethylene sulfide and hydrogen sulfide at room temperature provides ethanedithiol in 49% yield (E. M. J. Meade, et al., J. Chem. Soc., 1894 (1948)). The reaction of cyclohexene sulfide and potassium hydrogen sulfide provides cyclohexanedithiol in 38% yield (C. C. J. Culvenor, et al., J. Chem. Soc., 282 (1949)).